The project is concerned with the investigation of reactions of reactive intermediates with a potential use in photoaffinity labeling techniques. It addresses the need for development of new photoreactive reagents for studies of macromolecules important in many areas of medical and biological research. The understanding of reaction mechanisms of such reagents, which includes their kinetics and selectivity, is crucial to the optimization of the photolabeling. Such optimization should lead to more precise information about the interactions between small molecules of biological interest and macromolecules and macroassemblies such as enzymes, receptors and membranes. A simple test is proposed for the study of such photogenerated reactive intermediates. It allows one to estimate reaction rates of these species without absolute rate measurements. In this test two substrates with known diffusive properties and containing a different number of reactive sites compete for a reactive intermediate of interest. The approach uses diffusion rate as an internal clock for rapid reactions. The test is being used to estimate reaction rates of some of the least selective nitrenes and carbenes known. It provides data on C-H bond insertion, intersystem crossing, and complexation with solvents for these reactive intermediates.